Composite wires for coating substrates and methods of use

ABSTRACT

A composite wire for producing a wear resistant and corrosion resistant coating on a substrate by thermal spraying, spray and fuse, or welding techniques are disclosed. The physical properties of the coating are particularly suited for high-temperature erosion-corrosion environments. The resultant coating exhibits good hardness, toughness, and bonding characteristics. The composite wire comprises a metallic outer sheath and an inner core containing boron carbide and chrome carbide.

This application claims priority to U.S. Provisional Application No.60/476,875, filed on Jun. 6, 2003.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

FIELD OF THE INVENTION

The present invention relates to coatings for substrates, particularlymetallic substrates. More specifically, the invention relates tocomposite wires or cored wires for forming wear-resistant andcorrosion-resistant coatings on metallic substrates by thermal sprayingprocesses, spray and fuse processes, or by welding techniques. Theinvention also relates to methods of employing the composite or coredwires to apply wear-resistant and corrosion-resistant coatings tosubstrates.

BACKGROUND OF THE INVENTION

Thermal spraying, i.e. the generic name for a class of processes thatallow the depositing molten or semi-molten materials onto a substrate toform a wear or corrosion resistant coating, has been known in variousforms for many years. Thermal spraying processes include plasma, flame,arc-plasma, arc and combustion spraying. Arc spraying is a form ofthermal spraying which involves feeding two electrically conductivewires towards one another so that an arc is struck between the tips ofthe wires to melt the wire tips. The molten material is then atomizedand sprayed onto a substrate by compressed gas.

This form of thermal spraying is widely used to providecorrosion-resistant coatings on various metallic articles. For example,U.S. Pat. No. 6,156,443 (Dallaire, et al.) discloses coatings that areformed by arc spraying cored wires onto metallic articles exposed toerodent particles. The cored wires are composed of a sheath of ductilemetal, such as iron, low carbon steel, or ductile stainless steel, and acore comprising coarse ferroboron particles. The resulting coatings aredesigned to include iron boride phases having mean sizes equal to orlarger than the sizes of the erodent particles.

Another thermal spraying process is described in U.S. Pat. No. 5,837,326(Dallaire, et al.), which discloses a process for producing compositecoatings comprising fine titanium diboride particles or crystalsdistributed throughout a stainless steel matrix by arc spraying coredwires onto a metallic substrate. The cored wires typically comprise astainless steel outer sheath and an inner core of compacted powdersincluding titanium diboride and a metal or metal alloy. The particles orcrystals in these coatings impart hardness to the soft stainless steelmatrix and enhances the resistance of the coatings to hard abrasivemedia.

The metal components of the cored wires used to form the coatingsdisclosed in both U.S. Pat. Nos. 6,156,443 (Dallaire, et al.) and5,837,326 (Dallaire, et al.) are not highly alloyed, and consequentlythe coatings tend to be susceptible to corrosion attack in certain hightemperature corrosive conditions, such as in boiler applications.

Generally, the binder metal (i.e., the metal of the outer sheath) in awear-resistant coating is critical to the performance of the coating incorrosive conditions such as those encountered in boilers. For example,coatings with iron-based binder alloys, such as Armacor M™, exhibitextensive binder-material corrosion in boiler conditions, resulting inaccelerated wear of the coatings. The consequent weakening of the bondscan also lead to premature coating failure due to complete spalling ofthe protective layer. Furthermore, the magnetic characteristics of thesecoatings prevent thickness measurements using standard equipment, suchas Elcometers.

U.S. Pat. No. 4,741,974 (Longo, et al.) discloses a composite wire forforming wear resistant coatings wherein the wire is formed of an alloysheath having iron, nickel, or cobalt as a major component. The core ofthe composite wire is formed of powder that includes boron or boroncarbide. Due to its extreme hardness, boron carbide is employed incoatings where wear or abrasion is of primary concern. However, as withother conventional composite wires, in high-temperature corrosiveenvironments, the wear resistant coatings may experience acceleratedwear.

SUMMARY OF THE INVENTION

Accordingly, a composite wire is provided for producing a wear resistantand corrosion resistant coating on a substrate by thermal spraying,spray and fuse, or welding techniques. While the coating produced by thecomposite wire of the present invention is suitable for all temperatureranges, the physical properties of the coating are particularly wellsuited for high-temperature erosion-corrosion environments. Theresultant coating exhibits good hardness, toughness, and bondingcharacteristics. The composite wire generally comprises a metallic outersheath and an inner core containing boron carbide (BC) and chromecarbide (CrC). The metallic outer sheath may be formed of an essentiallypure metal or an alloy. As stated above, the binder is generallyconsidered to be critical to the performance of a coating and the binderselection will depend upon several factors including the environment towhich the coated substrate will be exposed. The composite wire of thepresent invention is manufactured by conventional techniques wherein thespecially formulated powder core is encapsulated by the outer sheath.The terms “composite wire” and “cored wire” are used synonymously hereinto describe such a wire.

The chrome carbide/boron carbide formulation of the inner core providesa substantial improvement to conventional coatings deposited byconventional composite wires. The combination of chrome carbide andboron carbide gives added wear resistance, increases metal binderhardness, improves bonding of the coating to the substrate, and inhibitsoxide formation.

The outer sheath may be constructed of any metal or alloy. Suitablebinder material includes, but is not limited to, iron, carbon and lowalloy steels, stainless steels, nickel, nickel alloys, such asnickel-copper, nickel-chromium, nickel-chrome-iron, andnickel-molybdenum alloys, copper, brasses, bronzes, aluminum bronzes,aluminum, aluminum alloys (1xxx-7xxx), titanium, titanium alloys, cobaltalloys, molybdenum and molybdenum alloys, tantalum and tantalum alloys.The combination of these binders with the inner core powder of thepresent invention results in coatings having superior physicalproperties over conventional coatings.

In addition to the composite wires discussed above, a method of forminga wear resistant and corrosion resistant coating on a substrate is alsoprovided. The method generally includes the steps of providing acomposite wire having and outer sheath formed from a metal or alloy, andan inner core comprising boron carbide and chrome carbide; and coating asubstrate by employing the composite wire in conjunction with thermalspraying, spray and fuse, or welding techniques.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is an electron micrograph of a cross section of an alloy 625 arcsprayed coating.

FIG. 2 is an electron micrograph of a cross section of an alloy625/BC-CrC arc sprayed coating.

PREFERRED EMBODIMENTS OF THE INVENTION

In the following detailed description of the preferred embodiments, itis to be understood that other embodiments may be utilized andstructural changes may be made without departing from the scope of thepresent invention.

The coatings according to the present invention are specificallydesigned for articles subjected to wear and/or corrosion. Such articlesinclude, for example, boiler tubes, hydraulic piston rods, pump casings,rollers in the paper and steel industry, wear plates, journals andshafts, and turbine blades and casings.

In one application, the coatings are designed to protect boiler tubesagainst erosion-corrosion related wastage and are applied to the boilertubes by means of a conventional arc spraying apparatus. However, itwill be appreciated from the description below that the coatings couldalso be applied to the boiler tubes by other thermal spraying apparatusemploying wires as the feed material, using fusing equipment, or bysuitable welding techniques. Arc spraying methods and apparatus are welldocumented in the art, see for example, U.S. Pat. Nos. 6,156,443(Dallaire, et al.); 5,837,326 (Dallaire, et al.); European Patent No. EP0 522 438 (Zurecki et al.); and PCT Patent No. WO 98/00574 (Seitz), thedisclosures of which are incorporated by reference.

The coatings of the present invention are formed from composite wires,which are fed through a conventional arc spraying apparatus, such as theapparatus disclosed in PCT Patent No. WO 01/08810 (Seitz), thedisclosure of which is incorporated herein by reference.

The composite wires of the present invention comprise an outer sheathformed from a metal or alloy. In one embodiment of the invention, whichis particularly suited for the high temperature erosion-corrosionenvironment found in boiler applications, the cored wires comprise anouter tubular sheath formed from a chromium bearing, nickel base alloysuch as an alloy 625, and an inner core, which comprises boron carbideand chrome carbide.

The preferred inner core formulation preferably comprises chrome carbidein an amount between about 25% and about 400% by weight of the amount ofboron carbide. In other words, the ratio of chrome carbide to boroncarbide preferably ranges between about 1:4 to about 4:1. Morepreferably, the inner core comprises chrome carbide in an amount betweenabout 67% and about 230% by weight of the amount of boron carbide. Inother words, the ratio of chrome carbide to boron carbide preferablyranges between about 1:1.5 to about 2.3:1. In one preferred embodiment,the inner core comprises about 100% by weight of chrome carbide relativeto the amount of boron carbide, in other words the amounts of chromecarbide and boron carbide are substantially equivalent.

The chrome carbide in the inner core increases the wear resistance ofthe deposited coating or weld overlay. The chrome carbide is retainedduring the thermal spraying or welding, and is present in molten formwithin the coating structure during application. However, this benefitaccrues on a sliding scale—the more chrome carbide used the better theresult, but at the expense of displacing boron carbide and itsbeneficial effects. Conversely, most of the boron carbide does notappear to survive as a carbide during application of the coating. Theboron carbide breaks up in the arc as uncontaminated boron and carbon,which both have substantial hardening effects. The hardening effectincreases the hardness of the metal of the outer sheath through alloyingand/or diffusion processes. Some of the larger particles of boroncarbide may survive the arc. Those particles add to the wear resistance,much like the chrome carbide, but the resistance achieved with largeparticles of boron carbide is not as effective as that achieved withchrome carbide. Furthermore, the boron carbide inhibits the formation ofoxides in the molten outer sheath during spraying. To achieve thisresult, a sufficient amount of boron carbide should be present in theinner core. Preferably, at least about 35% by weight of the inner coreshould consist of boron carbide to achieve very low oxide formation.However, this benefit accrues on a sliding scale—the more boron carbideused the better the result, but at the expense of displacing chromecarbide and its beneficial effects.

In addition to preventing oxide formation, the boron carbide also lowersthe melting temperature of the binder material (except for aluminum,which already has a low melting point) and improves the solidificationproperties of the metal matrix making up the deposited coating. Thisgives fusing and self-fluxing properties to the coating. Furthermore,there is an increase in the temperature range of the solid-liquid phase(i.e., during melting or freezing). The increased range improves thespray characteristics and also improves the fusing characteristics.

In addition to the qualities described above, the coatings of thepresent invention are also easier to install and maintain. The boronadditions in the metal binder matrix and the chrome carbide hard-phasemorphology make repairs to the coatings relatively easy. Added to this,the high bond strength and the low coating residual stresses allowexisting coatings to be grit blasted without disbonding the surroundingmaterial.

While the inner core formulation of the present invention centers aroundchrome carbide and boron carbide, the inner core may also containadditional materials. The additional materials may include: carbides,such as tungsten carbide, titanium carbide, vanadium carbide, and thelike; oxides, such as aluminum oxide, chrome oxide, zirconium oxide, andthe like; and borides, such as chrome boride, nickel boride, ironboride, and the like. The inner core may also include additional metalpowders, such as aluminum, nickel, chrome, or alloy powder, or compositepowders, such as tungsten carbide nickel and chrome carbide nickelchrome powders.

The grain size of the chromium carbide and boron carbide will have aneffect on the physical properties of the applied coating. Generally, thefiner the grains of the carbides, the more homogenous the coating willbe and generally the better the wear and corrosion properties. However,the cost and manufacturing constraints will limit the lower end of thegrain size range. U.S. Pat. No. 4,741,974 (Longo, et al.), thedisclosure of which is hereby incorporated by reference, discloses theeffect of grain size with respect to boron carbide.

EXAMPLES Example 1

A composite wire of the present invention was formed with an outersheath comprising of INCONEL® alloy 625 and an inner core comprising asubstantially equivalent mixture of chrome carbide and boron carbide.INCONEL® alloy 625 is a product of the Special Metals Corporation andcontains 0.1% C, 0.5% Mn, 5.0% Fe, 0.02% P, 0.015% S, 0.5% Si, 0.5% Cu,58% min Ni, 0.4% Al, 0.4% Ti, 20.0%-23.0% Cr, 3.15%-4.15% Nb+Ta, 8.0%10.0% Mo. This composite wire provided coatings with good toughnesscharacteristics, low oxides, and good wear characteristics due to thenovel carbide formulation of the inner core. In addition, there was anincrease in the hardness of the metal binder due to the boron content.The coatings also exhibited good bonding characteristics. The followingtable illustrates the improved wear resistance of a coatings depositedby a composite wire of the present invention (row 3 and row 4) comparedto the wear resistance of other coatings:

ASTM G65 Wear Test (15 minutes): Weight Loss (grams lost/6000 wheelrevolutions) Coating 1) Fe Cr B (Arc) 0.429 2) Fe Cr Mo W Mn B (HVOF)0.319 3) CrC/BC - Alloy 625 (Arc) 0.285 4) CrC/BC - Fe Cr (Arc) 0.212After Fusing 3) CrC/BC - Alloy 625 (Arc) 0.184 4) CrC/BC - Fe Cr (Arc)0.165

Note that coatings of the present invention, as discussed above, havelow oxide content and thus have superior fusing characteristics. Thewear resistances of the CrC/BC-Alloy 625 coating and the CrC/BC-Fe Crcoating were substantially improved upon fusing.

In addition to the improvement to wear resistance, the overall strengthof the metal binder was also substantially increased. The coatingdeposited with the CrC/BC-Alloy 625 composite wire exhibited an averagehardness measurement of 803.2 100 g/Vickers/10 seconds. Whereas,coatings of pure alloy 625 are typically on the order of 280-350 100g/Vickers/10 seconds.

Example 2

To illustrate bond strength, clean, bright smooth stainless steelcoupons were coated, without any surface roughness, to establish howthick a coating could be sprayed before spalling occurred (i.e., bondfailure). The first coupon was arc sprayed with pure alloy 625 and thesecond coupon was sprayed with a composite wire comprising an alloy 625outer sheath and an inner core comprising 40% by weight boron carbideand 60% by weight chromium carbide. The first coupon exhibiteddelamination at 300 μm, whereas the coating applied in accordance withthe present invention was sprayed up to 2000 μm without any appearanceof spalling.

Example 3

Two clean steel coupons were coated, with grit-blasted surfaces; thefirst with pure alloy 625 and the second with a composite wirecomprising an alloy 625 outer sheath and an inner core comprising 40% byweight boron carbide and 60% by weight chromium carbide. The couponswere mechanically damaged by cracking off the coating with a hammer andchisel. The pure 625 coating was peeled off of the first coupon as acontinuous layer, while the composite wire sprayed coating could not bedisjoined from the coupon. Rather, the composite wire sprayed coupononly sustained damage to the impact site, thus illustrating the highbond strength.

As stated above, in addition to good toughness, wear resistance,increased metal binder hardness, and improved bonding of the coating tothe substrate, the inner core formulation of the present invention alsoinhibits oxide formation. This characteristic of the boroncarbide/chrome carbide formulation is illustrated in FIGS. 1 and 2.

FIG. 1 shows a cross-sectional electron micrograph of a coating sprayedwith pure alloy 625. The coating exhibits high oxide content, whichshows up in the micrograph as grey bands. Due to the high oxide content,the alloy 625 coating will not fuse or melt as alloy 625 would fuse ormelt if it were oxide free. Conversely, as can be seen in FIG. 2, thecoating sprayed with an alloy 625/boron carbide-chrome carbide compositewire exhibits far less oxide content and can be easily remelted andfused.

While alloy 625 is a preferred alloy for the outer sheath in certainhigh temperature erosion-corrosion applications, alternative metals andalloys can also be employed. For example, alternative chrome bearingnickel base alloys include alloy C-276, alloy 686, or alloy 690.INCONEL® C-276, alloy 686, and alloy 690, which are all produced by theSpecial Metals Corporation contains: 0.02% C, 1.0% Mn, 4.0%-7.0% Fe,0.04% P, 0.03% S, 0.08% Si, 0.5% Cu, bal. Ni, 2.5% Co, 14.5%-16.5% Cr,15.0%-17.0% Mo, 3.0%-4.5% W (INCONEL® C-276); 0.01% C, 1.0% Mn, 5.0% Fe,0.02% P, 0.02% S, 0.08% Si, 0.5% Cu, bal. Ni, 0.5% Al, 0.25% Ti,19.0%23.0% Cr, 15.0%-17.0% Mo, 3.0%-4.4% W. (INCONEL® alloy 686); and0.02% C, 1.0% Mn, 7.0%-11.0% Fe, 0.015% S, 0.5% Si, 0.5% Cu, bal. Ni,27%-31% Cr (INCONEL® alloy 690). Nickel copper alloys, such as alloy400, alloy R-405, and the like, and nickel molybdenum alloys such as,alloy B, alloy B-2, and the like, may also be employed depending on therequired physical properties of the resulting coating and theenvironment to which the coating will be exposed.

As stated above, the metal binder material is not limited to nickel basealloys, rather the outer sheath may be constructed of any metal oralloy. Additional suitable binder material includes, but is not limitedto, iron, carbon and low alloy steels, stainless steels, nickel, copper,copper alloys (e.g., brasses, bronzes, and aluminum bronzes), aluminum,aluminum alloys (e.g., aluminum-copper, aluminum-manganese,aluminum-manganese-magnesium, aluminum-silicon,aluminum-manganese-magnesium-chrome, aluminum-magnesium-silicon, andaluminum-zinc-manganese-magnesium-copper), titanium, titanium alloys(e.g., titanium alloyed with palladium, molybdenum, nickel, aluminum,vanadium, niobium, tantalum, tin, zirconium, chromium and iron), cobalt,cobalt alloys (e.g., cobalt alloyed with chromium, nickel, molybdenum,and tungsten), zirconium, zirconium alloys, tantalum and tantalumalloys. The combination of any of these binders with the inner corepowder of the present invention results in coatings having superiorphysical properties over conventional coatings.

The cored wires may be formed in a conventional manner by placing themix of carbides, which need not be an agglomerated mix, onto an alloy625 strip, or a strip of some other outer sheath alloy, which is drawncontinuously through a plurality of wire drawing dies to form an outerwire sheath around an inner carbide core. The final outer diameter ofthe cored wire will depend upon the application for which it is used.For most applications, the cored wire final diameter ranges betweenabout 0.8 mm to about 6.4 mm. Conventional cored wire manufacturingtechniques are disclosed in U.S. Pat. application Nos. 6,156,443(Dallaire et al.) and 6,513,728 (Hughes et al.), of which bothdisclosures are hereby incorporated by reference.

Although the present invention has been described in terms of specificembodiments, it is anticipated that alterations and modificationsthereof will no doubt become apparent to those skilled in the art. It istherefore intended that the following claims be interpreted as coveringall alterations and modifications that fall within the true spirit andscope of the invention.

1. A composite wire for producing a wear resistant and corrosionresistant coating on a substrate, said composite wire consisting of atubular metallic outer sheath encapsulating a powdered inner core, saidinner core comprising a mixture of boron carbide of at least 35% byweight, and chrome carbide in an amount between about 67% and about 185%by weight of the amount of boron carbide.
 2. The composite wire of claim1, wherein the outer sheath is formed of an essentially pure metal. 3.The composite wire of claim 1, wherein the outer sheath is formed of analloy comprising a base metal.
 4. The composite wire of claim 3, whereinthe base metal of the alloy is selected from the group consisting ofiron, nickel, aluminum, molybdenum, tantalum, copper, and titanium. 5.The composite wire of claim 3, wherein the alloy is a nickel base alloy.6. The composite wire of claim 5, wherein the nickel base alloycomprises chromium.
 7. The composite wire of claim 6, wherein the nickelbase alloy comprises at least about 40% by weight of nickel.
 8. Thecomposite wire of claim 7, wherein the nickel base alloy furthercomprises molybdenum.
 9. The composite wire of claim 8, wherein thenickel base alloy is an alloy
 625. 10. The composite wire of claim 1,wherein the inner core comprises substantially equivalent amounts ofchrome carbide and boron carbide.
 11. The composite wire of claim 1,wherein the inner core further comprises another carbide in addition toboron carbide and chrome carbide.
 12. The composite wire of claim 1,wherein the inner core further comprises at least one carbide selectedfrom the group consisting of tungsten carbide, vanadium carbide andtitanium carbide.
 13. The composite wire of claim 1, wherein the innercore further comprises a metal powder.
 14. The composite wire of claim1, wherein the inner core further comprises a composite powder.
 15. Thecomposite wire of claim 14, wherein the inner core further comprisestungsten carbide nickel powder.
 16. The composite wire of claim 1,wherein the inner core further comprises a boride.
 17. The compositewire of claim 16, wherein the inner core further comprises at least oneboride selected from the group consisting of chrome boride, nickelboride, and iron boride.
 18. The composite wire of claim 1, wherein theinner core further comprises an oxide.
 19. The composite wire of claim18, wherein the inner core further comprises at least one oxide selectedfrom the group consisting of aluminum oxide, chrome oxide, and zirconiumoxide.